(i) Field of the Invention
The present invention relates to an in-building CATV system connected to a lead-in wire from an external bi-directional CATV system for transmitting downward signals inputted from the external bi-directional CATV system via an in-building transmission line to plural terminal devices in the building as well as for sending upward signals inputted from each end terminal to the external bi-directional CATV system via the lead-in wire.
(ii) Description of the Related Art
In a conventional known in-building CATV system, noises generated in the building are superimposed to the transmission line via subscriber end terminals and the like, and components of the noises with the same frequency as that of the upward signal are outputted to the external bi-directional CATV system as confluent noises together with the upward signal.
Heretofore, in order to decrease the confluent noises, the upward signal generated in the subscriber terminal device such as a cable modem (a signal with a frequency transmittable in the bi-directional CATV system: for example, 5 MHz-42 MHz) is frequency-converted to an upward signal (hereinafter referred to as an in-building upward signal) with a UHF band (for example, 913 MHz-950 MHz) which is higher than the original frequency and which does not overlap the frequency of the downward signal (for example, 54 MHz-890 MHz). The in-building upward signal is transmitted to the lead-in wire and is converted to the original frequency (that is, the transmission frequency of the upward signal in the bi-directional CATV system) using a down-converter, just before the upward signal is outputted to the lead-in wire from the in-building transmission line.
As mentioned above, when the upward signal is frequency-converted to the in-building upward signal at the terminal side of the in-building CATV system and the in-building upward signal is frequency-converted to the original upward signal at a connection point between the in-building CATV system and the external bi-directional CATV system, the confluent noises outputted to the external bi-directional CATV system from the in-building CATV system can be decreased. However when the up-converter used for frequency conversion at the terminal side and the down-converter used for frequency conversion at the connection point between the in-building CATV system and the external bi-directional CATV system are designed to frequency-convert using outputs from respective oscillation circuits for exclusive use, a deviation is generated in the frequency of a frequency-conversion reference signal in each oscillation circuit and so in some cases the in-building upward signal can not be converted to the original upward signal generated by the terminal device.
Therefore, in the conventional system, in order to restore the original upward signal from the in-building upward signal correctly in the down-converter, a pilot signal used for adjusting the level of the downward signal in the bi-directional CATV system is used as a reference signal which the up-converter and down-converter use for frequency conversion. Specifically, by constituting the up-converter and the down-converter to frequency-convert respective upward signals using the pilot signal of the bi-directional CATV system as a reference signal, the frequency of the reference signal for each converter is made to coincide with each other and the original upward signal is made to be perfectly restored from the in-building upward signal.
However, as mentioned above, if the up-converter and the down-converter are constituted to use the pilot signal contained in the downward signal as a reference signal for frequency conversion, the up-converter and the down-converter do not correctly operate when transmission of the pilot signal is stopped in the bi-directional CATV system for maintenance, inspection or the like, or when the in-building CATV system is connected to a bi-directional CATV system which does not transmit pilot signals. Therefore, the upward signal generated in the terminal devices of the in-building CATV system can not be transmitted to the center equipment of the bi-directional CATV system.